A characteristic of lithium batteries is that should an over-current condition cause overheating, the temperature rise of the battery exacerbates the current rise and a runaway condition can occur. It is commonplace to require that such batteries be provided with thermal protectors which open the battery circuit in the event of a temperature rise beyond a set value.
It is known in fields other than battery protection that an alarm may be triggered when a limit temperature is reached by using a fusible element which holds a switch open at low temperatures and which melts at a limit temperature to permit the switch to close to activate an alarm. Examples of these devices are given by the U.S. Pat. No. 4,144,532 to Boyd for a Thaw Alarm for Food Freezer and the U.S. Pat. No. 4,771,270 to Kelso for a Temperature Sensitive Fire Alarm Unit. The former patent uses a congealed water-alcohol mixture to hold switch contacts apart and the latter uses a fusible cermet strip for the same purpose.
Prior devices for battery thermal protection include a small sensor subject to only one cell of a battery so that protection is sometimes denied in conditions where one cell in a battery is overheating. Those sensors also are subject to self heating by the battery current so that the temperature of the device reflects the current load as well as the cell temperature and can trigger a shut down before the temperature limit is reached. This is a particular problem where an application requires large current pulses which heat the sensor much more than the battery.